1. Field of the Invention
The present invention relates to a process for producing diol compounds, for example, 1,6-hexanediol, 1,5-pentanediol and 1,4-butanediol by esterifying a carboxylic acid mixture collected from a reaction product mixture liquid of a liquid phase oxidation of cyclohexane with, for example, air, and then hydrogenate-decomposing the resultant esterification product. These diol compounds are useful as polyurethane elastomers, as additives for synthetic resins and as intermediates of medicines and agricultural chemicals.
2. Description of Related Art
As disclosed in Japanese Examined Patent Publication Nos. 49-27,164 and 53-33,567, cyclohexanol and cyclohexanone, which are useful as materials for the synthesis of .epsilon.-caprolactam, are industrially produced by a liquid phase oxidation of cyclohexane with air, and diols, for example, 1,6-hexanediol, are produced by esterifying a carboxylic acid mixture obtained as a by-product of the above-mentioned oxidation reaction with an alcohol, and then hydrogenate-decomposing the resultant esterification product with hydrogen.
In the conventional process for producing diol compounds, a copper-chromium-containing catalyst is commonly employed as a hydrogenation catalyst.
The conventional copper-chromium-containing catalyst contains chromium, which is harmful, and thus when this catalyst is used, specific dust-preventing means are necessary while handling the catalyst. Also, the conventional chromium-containing catalyst is disadvantageous in that specific equipment is necessary for the treatment of discharged water and waste liquid. Especially, where the reaction is carried out in a liquid phase suspension state, since the catalyst partially dissolves in the reaction liquid, it is difficult to treat the residue in a distillation column after the diol compounds, for example, 1,6-hexanediol, are collected from the reaction product mixture liquid by distillation.
Accordingly, various chromium-free catalysts have been evaluated for the process for producing alcohol compounds by hydrogenate-decomposing carboxylic acid esters with hydrogen. However, these conventional catalysts are not industrially satisfactory for the production of diol compounds, for example, 1,6-hexanediol.
For example, Japanese Examined Patent Publication (Kokoku) No. 58-4 50775 discloses a process for producing alcohols from corresponding methyl esters of coconut oil fatty acids by using a catalyst comprising copper oxide and iron oxide carried on aluminum oxide. However, when this catalyst is applied to the production of 1,6-hexanediol, this catalyst exhibits a significantly lower activity than that of the conventional copper-chromium-containing catalyst, whereas the filter-separability of this catalyst is similar to that of the conventional copper-chromium-containing catalyst.
Also, Japanese Unexamined Patent Publication (Kokai) No. 63-141937 discloses a process for producing lauryl alcohol from methyl laurate by using a catalyst consisting of copper oxide and zinc oxide. When this catalyst is applied to the production of 1,6-hexanediol, however, this catalyst has the disadvantage that its filter-separability is very poor, whereas the activity of the catalyst is higher than that of the conventional copper chromium-containing catalyst.
Further, Japanese Unexamined Patent Publication (Kokai) No. 61-2016 discloses a copper and zinc-containing hydrogenation catalyst usable for the production of ethylene glycol. This catalyst is prepared by adding an alkali solution to an aqueous solution containing copper ions and zinc ions at room temperature at a pH of 3 to 7, calcining the resultant precipitate and reducing the calcining product. When this type of catalyst is applied to the preparation of diol compounds, it has the disadvantage that its filter-separability is poor.
Still further, Japanese Unexamined Patent Publication (Kokai) No. 57-53421 discloses a process for producing propane diol by hydrogenating hydroxypropionaldehyde in the presence of a hydrogenation catalyst prepared by precipitating a crystal mixture containing a copper-zinc complex of the empirical formula: EQU Cu.sub.1.5-3 Zn.sub.1-2.5 (CO.sub.3).sub.1-2 (OH).sub.4-6 (H.sub.2 O).sub.0-1
and aluminium compounds, from an aqueous solution of copper and zinc compounds and an aluminum compound in the presence of a carbonate at a pH of 6.9 to 8.0, and heat-decomposing the crystal mixture at a temperature of 200.degree. C. to 500.degree. C. This catalyst is used in the form of pellets. However, it is not known whether or not this type of catalyst is usable for the production of diol compounds from a carboxylic acid-esterification product. Still further, it is known that a chromium-free hydrogenation catalyst comprising metallic copper, copper oxide and zinc oxide is produced by reducing a mixture of copper oxide and zinc oxide. This type of catalyst is usable for the preparation of methyl alcohol from a synthetic gas, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-26526, and for the synthesis of cumene and ethylbenzene from methylstyrene and acetophenone, as disclosed in DD 218090. This type of catalyst is disadvantageous in that when employed for the production of diol compounds, the catalyst exhibits a poor filter-separability. Also, this type of catalyst commonly exhibits a high reactivity and easily generates heat or ignites when brought into contact with air. Therefore, this catalyst is very difficult and complicated to handle safely.